I have read and re-read the message from Mr. Jobs. At no point does he say that he instructed Mr. Shiller to give the Apple keynote at Macworld due to his health concerns. He remarks only that the order sparked another round of rumors about his health.
Now, you may certainly speculate that Mr. Jobs has decided to sit out Macworld due to his health, but that should be reported as the speculation it is -- not as a matter of fact -- as the record provided by Apple and Mr. Jobs does not support it.
The letter pretty much makes the connection between Schiller getting the keynote and Steve making the decision based on his health.